There have been widely used as power supply units switching DC-DC converters for converting an input voltage to a predetermined output voltage through switching of the input voltage.
These DC-DC converters have high power conversion efficiency as compared with series regulators. For this reason, many of portable apparatuses such as mobile phones, personal computers, and PDAs powered by a battery utilizes a DC-DC converter as a power supply unit for acquiring a predetermined operating voltage different from the battery voltage.
FIG. 3 illustrates a conventional DC-DC converter 40. The input source voltage (referred to as battery voltage) Vbat supplied from a battery BAT is converted by the DC-DC converter 40 into a predetermined output voltage Vo. Besides the battery voltage Vbat is used to generate the output voltage Vo, it is also used by various circuit blocks 31-3n of the portable apparatus as their operating voltages.
The battery voltage Vbat may be also converted by a series regulator to a first predetermined voltage to be supplied as an operating voltage of a predetermined circuit block. Japanese Patent Application Laid Open No. H08-331036 discloses a further switching power supply circuit for converting the first predetermined voltage to a second predetermined voltage to be supplied to other circuit blocks.
However, in the DC-DC converter as disclosed in the cited document, switching noise arising from the switching operation in the converter appears on the power source line. If this switching noise is superimposed on the source voltage, the superposed noise will become a cause of adverse influence on other circuit blocks built together with the DC-DC converter in the same IC.
For example, the switching noise can result in erroneous data or deteriorate the linear characteristic of the converter, thereby lowering the overall reliability and deteriorating characteristics of the IC. The switching noise can further disadvantageously affect the operation of the portable apparatus if it is equipped with the IC.
In order to reduce the influence of the switching noise, one might use a filter circuit consisting of a resistor and a capacitor. However, such filter circuit increases power loss by the resistor when the load is heavy.